The main reason is forgetfulness but others admit they deliberately skip taking the medication because they feel they don't need it any more and feel better.

One in 20 were anxious about side-effects while a minority believed the medication was not doing them any good.

The findings in a "Report on Patient Non-Adherence in Ireland" showed that, overall, more than one in two carers confirmed the person they were looking after regularly forgot to take their medication.

Overall, 18pc of those surveyed said they were failing not taking the medications all of the time but it was as high as 23pc in men and women among the under-35s.

Among those who "sometimes" did not take the medication, as many as 64pc did not do so "one or more times a week", the survey commissioned by the drug company Pfizer, the Irish Pharmacy Union and the Irish Patients' Association found.

Those most likely to take medication are patients who speak regularly to their doctor, have a good grasp of their condition and have an understanding of the treatment.

The survey of 1,003 adults aged 16 and over highlighted that more than one-quarter of participants suffered from a listed condition.

It called for better access to training for healthcare professionals and more support for the patient in managing their health.

Commenting on the findings, Rory O'Donnell, president of the Irish Pharmacy Union, said: "Pharmacists are among the most accessible and most consulted healthcare professionals and are therefore ideally placed to tackle non-adherence to medication.

"They can educate patients, help them make informed decisions about their medicines and support them in adhering to their prescribed therapy."

Stephen McMahon, chief executive of the Irish Patients' Association, said it was evident that all those involved with the patient – including the doctor, the pharmacist, the nurse, the payer, the policy maker and the manufacturer – should play a role in promoting the importance of adhering to prescribed medication.